Downing Street has failed to match Boris Johnson’s enthusiasm for a 22-mile bridge across the Channel, saying there were no plans for such a project.

Theresa May’s official spokesman repeatedly declined to offer support for the idea but stressed that the prime minister and foreign secretary both backed an agreement to have a panel of experts looking at major projects together with France.

Pressed several times on whether the prime minister would be open to the idea of a cross-Channel bridge, the spokesman said there were “no specific plans”.

“What was agreed yesterday, and I think that’s what the foreign secretary tweeted about as well, is a panel of experts who will look at major projects together including infrastructure,” he said.

Quick guide

All you need to know about Anglo-French trade

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Which country is 'on top'?

Roughly £71bn of goods and services were traded between the two countries in 2016. France has the upper hand: the UK exported £33.8bn to France but imported £37.6bn. Exports to France have fallen by about 9% over the last decade, while imports are roughly flat. France is Britain's third-largest export market.

What gets traded?

There is an appreciation on both sides of the Channel for what each country does well: Britain is the largest importer of champagne, while more than 28m Harry Potter books have gone the other way. France is the second biggest European food exporter to the UK and accounts for 20% of dairy imports. There were more than 500 French restaurants in Britain in 2017, 54 of them in the Michelin Guide. Among the most common UK exports are cars, chemicals and financial services. France is a big exporter of aircraft, machinery and cars.

Living and working

About 150,000 British citizens live in France, while 155,000 French nationals are settled in the UK. Banking is the most common type of employment for French people in Britain, with the vast majority of them living in London and the south-east; there are 15 accredited French schools in the UK, 13 of which are in London. Roughly a quarter of all British citizens in France live in the Nouvelle-Aquitaine region in western France.

Tourism

The French make about 4m visits a year to the UK, making them the number one nationality of foreign visitors. About 11 million tourists visit France every year from the UK, more than from any other country.

Business links

More than 1,000 subsidiaries of British companies were based in France in 2014, generating 195,000 jobs. French companies with major operations in Britain include the energy giant EDF and the utilities firm Veolia.

France’s finance minister, Bruno Le Maire, also appeared to be lukewarm about the concept of a bridge.

“All ideas merit consideration, even the most far-fetched ones,” Le Maire told Europe 1 radio. “We have major European infrastructure projects that are complicated to finance. Let’s finish things that are already under way before thinking of new ones.”

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Johnson’s suggestion of a bridge or underground road link to improve post-Brexit transport links with France dominated the reporting of May’s summit with Macron and their ministers.

The foreign secretary went directly to Macron to discuss the proposal, saying it was “ridiculous” that two of the world’s biggest economies were linked by a single railway line. A source at the summit said Macron responded positively with an agreement that a second link should be built.

Later on Thursday, Johnson tweeted a picture of himself and Macron both giving a thumbs-up sign, captioned: “En marche! Great meetings with French counterparts today.”

He also tweeted: “I’m especially pleased we are establishing a panel of experts to look at major projects together. Our economic success depends on good infrastructure and good connections. Should the Channel tunnel be just a first step?”

The suggestion was immediately met with scepticism by the UK Chamber of Shipping, which represents more than 180 maritime industry bodies.

“Building a huge concrete structure in the middle of the world’s busiest shipping lane might come with some challenges,” it tweeted, though the body might be expected to take a dim view of any potential maritime obstruction in the Channel.

Johnson’s Conservative colleague Charlie Elphicke, the MP for Dover, said cash should be invested in Kent instead. “Boris is right. We absolutely must invest in infrastructure to keep trade flowing between Britain and France,” he said.

“Let’s start by dualling the A2 to Dover, and building the Lower Thames Crossing and lorry parks on the M20. Surely it’s not a bridge too far for the government to invest in Kent.”

Engineers, however, have said the idea of a Channel bridge was not as far-fetched as it may seem.

Ian Firth, a bridge designer and a former president of the Institution of Structural Engineers, told the BBC: “It is entirely feasible. Before the tunnel was built there were bridge options being looked at. There are bridges of a similar, if not quite the same, scale elsewhere. Of course this would not be one big span; the economics may lean towards something like 800m to 1km spans.

“It would be a huge undertaking, but it would be absolutely possible, and shipping impact issues could be dealt with.”

Dave Parker, technical editor of New Civil Engineer magazine, told the programme that problems with shipping could be avoided by building artificial islands in the Channel that were linked to the shore by viaducts and joined to one another by a tunnel.

The islands could become venues for attractions like hotels, casinos and duty-free shops, he suggested.

Johnson has previously promoted the idea of another Channel tunnel but is now said to think a bridge could also be possible, telling aides that such feats of engineering have been achieved in Japan.

Boris Johnson's errors of judgment

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Nazanin Zaghari-Ratcliffe

Boris Johnson said that the British-Iranian citizen Nazanin Zaghari-Ratcliffe, convicted of spying in Iran, was “simply teaching people journalism” – a statement her family and her employer both said was untrue. His comments were subsequently cited as proof that she was engaged in “propaganda against the regime”.

'Dead bodies'

After Johnson suggested that the Libyan city of Sirte might become a new Dubai once “the dead bodies” were removed, Downing Street said it was not “an appropriate choice of words”.

Johnson apologised after causing a “livid” reaction in a worshipper in a Sikh temple in Bristol by discussing his enthusiasm for ending tariffs on whisky traded between the UK and India. Alcohol is forbidden under some Sikh teachings.

Continental drift

Boris Johnson referred to Africa as “that country” in his Conservative party conference speech.

Johnson has championed an array of ambitious infrastructure projects that have not come to fruition. As mayor of London, he opened the £60m Emirates Air Line cable car that has failed to attract hoped-for commuter traffic.